: The ventilated lung is injured during ischemia, prior to reperfusion. This injury is manifest as increased pulmonary vascular permeability. The mechanisms underlying ischemic lung injury are not well understood, but are likely to differ from systemic organs, as pulmonary ischemia is not synonymous with hypoxia if ventilation is maintained when blood flow is impaired. Vascular endothelial growth factor is a potent endothelial cell-specific mitogen, also called vascular permeability factor because of its ability to induce vascular leakage of plasma proteins. The mitogenic effects of VEGF require the induction of its endothelial cell tyrosine kinase receptors, fms-like tyrosine kinase (flt-1) and kinase insert domain-containing receptor/fetal liver kinase (KDR/flk-1), though it is not known if these receptors mediate its permeability-enhancing effects. Tissue levels of VEGF and its receptors increase in vivo during myocardial and cerebral ischemia, presumably in response to tissue hypoxia. In vitro experiments suggest that VEGF is also upregulated by reactive oxygen species, inhibition of nitric oxide, and glucose depletion, all stimuli which may be present in the ventilated ischemia lung. VEGF induction may be an important protective response to ischemia if angiogenesis is stimulated, but could also have adverse consequences on vascular permeability. The experiments outlined in this proposal will determine whether VEGF and its receptors are upregulared during ventilated pulmonary ischemia, and whether VEGF contributes to the development of vascular leak in the ischemic lung. VEGF mRNA and protein will be measured as a function of ischemic time and oxygen tension during ventilated pulmonary ischemia, and VEGF protein will be localized using immunohistochemistry. These findings will be correlated with vascular permeability, measured by both physiologic assays of protein permeability, and morphologic assessment of endothelial leak by vascular labeling techniques, and evaluation of endothelial morphology by electron microscopy. Additionally, we will determine whether inhibition of VEGF attenuates ischemic lung injury. Finally, studies will be performed in vitro to determine cellular pathways by which VEGF increases endothelial permeability. These studies may lead to novel strategies to decrease pulmonary ischemic injury, such as occurs with lung preservation for transplantation, and will further our understanding of the mechanisms which regulate vascular barrier function in the lung.